The weight of a letter has factored into the required amount of postage since 1845. Many customers wait until they reach the post office to find out the weight and the expense of mailing their items. Others seek the knowledge and convenience afforded by scales made for personal and business mailing needs. Commercial scales help patrons save expenses of time and money. Scales on the market vary by their intended purposes, from the quick-use, pocket versions primarily for households to larger and more accurate ones for high-volume business mailing.
Lynn Heidelbaugh, National Postal Museum